Sintered porous filters made from thermoplastic materials are used extensively in treating potable water and other fluids to remove particulate and dissolved contaminants. (See U.S. Pat. No. 6,030,558; U.S. Pat. No. 4,797,243; U.S. Pat. No. 5,547,481; EPO Patent 743,085A2; and EPO Patent Application 659,482A1.) Sintering is a process of compressing powdered thermoplastics in a mold at temperatures just below their melting point. Powdered particles are fused together but the mass, as a whole does not melt. These sintered materials are used as filters since they can be made to have specific porosity. These filters are typically made by fusing discrete particles of selected thermoplastic polymers under elevated temperature and pressure conditions while enclosed in a mold. These thermoplastic polymers typically have a low melt flow rate, which means that at their melting point they have a very high viscosity which minimizes the flow of the melted mass. When heated, the polymer particles fuse at the point of contact, creating a solidified porous body with a complex pore structure and with good mechanical strength. It is necessary that the particles retain their shape, save for the slight softening at the point of contact where fusing occurs as the temperature is reduced from the elevated temperatures to near ambient levels. Particle shape retention can occur only when the melt flow rate of the polymer is low. The melt flow rate is defined by the so-called Melt Index, which is a measure of the amount of material that is extruded from a small orifice during a period of 10 minutes at 43.5 psi of pressure. Sintering temperatures of 374F. (190C.) and 446F. (230C.) are specified for polyethylene and polypropylene respectively. (ASTM Method-D-1238). A high melt index material indicates a low-viscosity polymer. Melt Index is calculated and given as grams per 10 min. Low melt index is usually achieved when the molecular weight of the polymer is high. One of the most suitable thermoplastic polymers used for these filters is High Density Polyethylene (HDPE) with a molecular weight approaching one million. So-called Ultra High Density Polyethylenes have molecular weights of one million or more and have melt indices of 0.0 to 0.5.
Any thermoplastic polymer meeting the desired low melt index conditions can be used for filter applications, including, without limitation, polypropylene, polyethylene, polysulfone, polyethersulfone, polyphenylene sulfide, ethylene vinyl acetate and the like. Typically, any thermoplastic polymer can be irradiated with either gamma rays or X-rays to induce cross-linking within it, which results in an increase in its molecular weight and which decreases its melt flow rate. Typically, sintered porous filters are made with predetermined particle size, which determines the size of its pores; i.e., the finer the size of the original particle, the finer is the average size of the pore in the sintered product. In fluid filtration applications, sintered filters have average pore sizes from 5 microns to 100 microns. The original size of the thermoplastic particle to produce these pore sizes in the filter is from 40 to 800 microns. More typically, the particle size is from 75 to 300 microns. Besides the size of the original particles, the porosity of the sintered filter can also be controlled by using blends of high and low melt flow materials. In this case, high melt polymers determine the average pore size, while the low melt polymer gives the filter its structural strength. Besides using blends of similar and dissimilar polymers with high and low melt flow rate, it is also possible to add other types of particulate materials in the matrix that impart other properties to the filter structure. For instance it is possible to embed activated carbon, synthetic ion exchange resins, inorganic and organic adsorption media such as metal oxides, modified peat, etc., in the sintered product (See EP Patent 0,659,482A1). The resultant filter then has the added capability to adsorb volatile organic substances such as pesticides, inorganic dissolved ions such as calcium, lead, mercury, arsenic and nitrates, etc., besides its traditional function to remove particulate materials.
Although the science and art of making sintered filters with various capabilities to remove particulate material and dissolved organic and inorganic impurities from fluid is well established, there is still a paucity of technology to protect these sintered porous filters from microbial growth. Trapped microorganisms are able to proliferate in these filters, creating serious health hazards, especially in the field of drinking water and where the filters are used in the processing of materials for human consumption such as food. The growth of microorganisms within the filter also reduces flow through the filter and requires higher operating pressures and/or frequent filter replacement. One of the technologies to prevent the growth of microorganism uses iodinated synthetic ion exchange resin embedded in sintered thermoplastic porous filter (See EP 0,659,482 A1“Ion exchange resin sintered in porous matrix” by Edward C. Giordano and Hans-Gunther Sternagel) Here antimicrobial action depends on slow release of iodine ions. While this method is acceptable to the U.S. Environmental Protection Agency (EPA) on a temporary or emergency basis to purify drinking water, its prolonged usage is not approved by the EPA because of the adverse effect of iodine on human health. In other known technology a synthetic ion exchange resin or other material such as activated carbon can be treated with silver and then subsequently embedded in the porous sintered thermoplastic filter. This method of treatment suffers from very high cost and also by the fact that silver surfaces become deactivated when exposed to water containing dissolved chlorine or chloride ions.
Applicant is aware of the following U.S. Patents and European Patent Office publications concerning the use of sintered porous filters made from thermoplastic materials.
U.S. Pat. No.InventorIssue DateTitle6,030,558Smith et al.Feb. 29, 2000Sintered Porous PlasticProducts and Methodof Making Same5,547,481Herding et al.Aug. 20, 1996Filter Element HavingAn Inherently Stable,Permeably PorousPlastic Body4,797,243WolbromJan. 10, 1989Dye-ContainingPorous Plastic Elementand Method ofMaking ItEPOInventorPub. DateTitle0743085A2Takiguchi et al.Nov. 20, 1996Porous Plastic Filterand Process For ItsProduction0659482A1Giordano et al.Jun. 28, 1995Ion Exchange ResinSintered In PorousMatrix